Re: [External] Re: [PATCH bpf-next v2 5/6] bpf: teach the verifier to enforce css_iter and process_iter in RCU CS

[Chuyi Zhou]

Hello.

在 2023/9/15 07:26, Andrii Nakryiko 写道:
> On Thu, Sep 14, 2023 at 1:56 AM Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx> wrote:
> > 在 2023/9/13 21:53, Chuyi Zhou 写道:
> > > Hello.
> > >
> > > 在 2023/9/12 15:01, Chuyi Zhou 写道:
> > > > css_iter and process_iter should be used in rcu section. Specifically, in
> > > > sleepable progs explicit bpf_rcu_read_lock() is needed before use these
> > > > iters. In normal bpf progs that have implicit rcu_read_lock(), it's OK to
> > > > use them directly.
> > > >
> > > > This patch checks whether we are in rcu cs before we want to invoke
> > > > bpf_iter_process_new and bpf_iter_css_{pre, post}_new in
> > > > mark_stack_slots_iter(). If the rcu protection is guaranteed, we would
> > > > let st->type = PTR_TO_STACK | MEM_RCU. is_iter_reg_valid_init() will
> > > > reject if reg->type is UNTRUSTED.
> > >
> > > I use the following BPF Prog to test this patch:
> > >
> > > SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
> > > int iter_task_for_each_sleep(void *ctx)
> > > {
> > >       struct task_struct *task;
> > >       struct task_struct *cur_task = bpf_get_current_task_btf();
> > >
> > >       if (cur_task->pid != target_pid)
> > >           return 0;
> > >       bpf_rcu_read_lock();
> > >       bpf_for_each(process, task) {
> > >           bpf_rcu_read_unlock();
> > >           if (task->pid == target_pid)
> > >               process_cnt += 1;
> > >           bpf_rcu_read_lock();
> > >       }
> > >       bpf_rcu_read_unlock();
> > >       return 0;
> > > }
> > >
> > > Unfortunately, we can pass the verifier.
> > >
> > > Then I add some printk-messages before setting/clearing state to help
> > > debug:
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index d151e6b43a5f..35f3fa9471a9 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -1200,7 +1200,7 @@ static int mark_stack_slots_iter(struct
> > > bpf_verifier_env *env,
> > >                   __mark_reg_known_zero(st);
> > >                   st->type = PTR_TO_STACK; /* we don't have dedicated reg
> > > type */
> > >                   if (is_iter_need_rcu(meta)) {
> > > +                       printk("mark reg_addr : %px", st);
> > >                           if (in_rcu_cs(env))
> > >                                   st->type |= MEM_RCU;
> > >                           else
> > > @@ -11472,8 +11472,8 @@ static int check_kfunc_call(struct
> > > bpf_verifier_env *env, struct bpf_insn *insn,
> > >                           return -EINVAL;
> > >                   } else if (rcu_unlock) {
> > >                           bpf_for_each_reg_in_vstate(env->cur_state,
> > > state, reg, ({
> > > +                               printk("clear reg_addr : %px MEM_RCU :
> > > %d PTR_UNTRUSTED : %d\n ", reg, reg->type & MEM_RCU, reg->type &
> > > PTR_UNTRUSTED);
> > >                                   if (reg->type & MEM_RCU) {
> > > -                                       printk("clear reg addr : %lld",
> > > reg);
> > >                                           reg->type &= ~(MEM_RCU |
> > > PTR_MAYBE_NULL);
> > >                                           reg->type |= PTR_UNTRUSTED;
> > >                                   }
> > >
> > >
> > > The demsg log:
> > >
> > > [  393.705324] mark reg_addr : ffff88814e40e200
> > >
> > > [  393.706883] clear reg_addr : ffff88814d5f8000 MEM_RCU : 0
> > > PTR_UNTRUSTED : 0
> > >
> > > [  393.707353] clear reg_addr : ffff88814d5f8078 MEM_RCU : 0
> > > PTR_UNTRUSTED : 0
> > >
> > > [  393.708099] clear reg_addr : ffff88814d5f80f0 MEM_RCU : 0
> > > PTR_UNTRUSTED : 0
> > > ....
> > > ....
> > >
> > > I didn't see ffff88814e40e200 is cleared as expected because
> > > bpf_for_each_reg_in_vstate didn't find it.
> > >
> > > It seems when we are doing bpf_read_unlock() in the middle of iteration
> > > and want to clearing state through bpf_for_each_reg_in_vstate, we can
> > > not find the previous reg which we marked MEM_RCU/PTR_UNTRUSTED in
> > > mark_stack_slots_iter().
> >
> > bpf_get_spilled_reg will skip slots if they are not STACK_SPILL, but in
> > mark_stack_slots_iter() we has marked the slots *STACK_ITER*
> >
> > With the following change, everything seems work OK.
> >
> > diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> > index a3236651ec64..83c5ecccadb4 100644
> > --- a/include/linux/bpf_verifier.h
> > +++ b/include/linux/bpf_verifier.h
> > @@ -387,7 +387,7 @@ struct bpf_verifier_state {
> >
> >    #define bpf_get_spilled_reg(slot, frame)                               \
> >           (((slot < frame->allocated_stack / BPF_REG_SIZE) &&             \
> > -         (frame->stack[slot].slot_type[0] == STACK_SPILL))             \
> > +         (frame->stack[slot].slot_type[0] == STACK_SPILL ||
> > frame->stack[slot].slot_type[0] == STACK_ITER))            \
> >            ? &frame->stack[slot].spilled_ptr : NULL)
> >
> > I am not sure whether this would harm some logic implicitly when using
> > bpf_get_spilled_reg/bpf_for_each_spilled_reg in other place. If so,
> > maybe we should add a extra parameter to control the picking behaviour.
> >
> > #define bpf_get_spilled_reg(slot, frame, stack_type)
> >                          \
> >          (((slot < frame->allocated_stack / BPF_REG_SIZE) &&             \
> >            (frame->stack[slot].slot_type[0] == stack_type))              \
> >           ? &frame->stack[slot].spilled_ptr : NULL)
> >
> > Thanks.
>
> I don't think it's safe to just make bpf_get_spilled_reg, and
> subsequently bpf_for_each_reg_in_vstate and bpf_for_each_spilled_reg
> just suddenly start iterating iterator states and/or dynptrs. At least
> some of existing uses of those assume they are really working just
> with registers.

IIUC, when we are doing bpf_rcu_unlock, we do need to clear the state of 
reg including STACK_ITER.

Maybe here we only need change the logic when using 
bpf_for_each_reg_in_vstate to clear state in bpf_rcu_unlock and keep 
everything else unchanged ?

Thanks.